1. Field of the Invention
This invention relates to a temperature fixed-point cell and a temperature fixed-point device required for the calibration of such thermometers as radiation thermometer and thermocouple that are used in the high-temperature range exceeding 1000° C.
2. Description of the Prior Art
In the calibration of a thermometer above the normal room temperature, the freezing point or the melting point of a metal is adopted as the defining fixed point and the fixed-point cell is used as a means to realize the temperature scale. This means is generally a crucible that is made of graphite having a pure metal cast therein. The calibration of the thermometer is accomplished by measuring the temperature of the interior of the cell with the thermometer, placing the fixed-point cell in a temperature-variable furnace, observing the change of temperature of the cell taking place when the ambient temperature is increased or decreased, and utilizing the phenomenon that the change of temperature is caused to vanish by the latent heat of melting or freezing in the state where the liquid phase and the solid phase coexist (refer to “Newly Compiled Edition: Correct Way to Use of Thermometer” edited by (Juridical Person) Japan Electrical Measuring Instrument Industry Society, Chapter 7, published by Japan Industrial Publishing Co., Ltd. (1997)).
The highest temperature of the defining fixed point is the copper point, 1085° C. In the temperature range exceeding the copper point, the temperature scale is defined by extrapolation. In the temperature range exceeding the copper point, the scale is maintained by utilizing a radiation thermometer that has undergone fixed-point calibration below the copper point or by utilizing the relation between the radiance and the electrical current of a filament lamp using a tungsten ribbon as the filament.
An idea of using the freezing point of palladium (1550° C.) or platinum (1770° C.) with a view to realizing a fixed point in the temperature range exceeding the copper point has been tried and experiments of measuring the fixed-point temperatures by melting such a material in a crucible made of alumina has been reported (refer to Quinn, T. J., Chandler, T. R D.: Temperature, Its Measurement and Control in Science and Industry, Plumb, H. H. (ed.), Vol. 4, Part 1, p. 295, Pittsburgh: Instrument Society of America (1972) and Coates, P. B., Chandler, T. R D., Andrews, J. W., High Temperature and High Pressure, Vol. 15, p. 573(1983)).
An idea of using tungsten as the material for a crucible, melting alumina in the crucible, observing the melting and the freezing of alumina at 2050° C. with a radiation thermometer, and adopting the melting point as the fixed point has been tried and reported (refer to Sakate, H., Sakuma, F., Ono, A., Metrologia, Vol. 32, p. 129, (1995)).
In the calibration of a thermocouple, the practice of resorting to the calibration by the palladium wire method in addition to the calibration at the copper point (1085° C.) or the gold point (1064° C.) is commonly practiced. This is a method that consists in inserting a purely metallic palladium wire into the leading terminal of a thermocouple, increasing the temperature of the wire in a heating furnace, and observing the temperature at which the wire temperature, while the wire melts, assumes a state of plateau.
On the other hand, an idea of using a metal-metal eutectic alloy for a temperature fixed point has been tried. For example, an idea of realizing a fixed point by casting a copper-silver eutectic alloy or a copper-aluminum eutectic alloy in a crucible made of graphite and observing the melting and the freezing of the eutectic alloy has been reported (refer to Ito, Transaction of The Society of Instrumentation and Control Engineers, Vol. 19, No. 12, p. 978 (1983)).
Then, a temperature fixed-point cell that is formed of a crucible that is made of graphite and using a metal-carbon eutectic alloy as a fixed-point material and enclosing the fixed-point material therein has been disclosed (refer to Japanese Patent No. 2987459). This invention enables realizing a plurality of temperature fixed points between 1100° C. and 2500° C.
Further, a temperature fixed-point cell that is formed of a crucible made of graphite and using a metal carbide-carbon eutectic alloy as a fixed-point material and enclosing the fixed-point material therein has been disclosed (refer to Japanese Patent No. 3404531). This invention enables realizing a plurality of temperature fixed points between 2500° C. and 3200° C.
The fixed point of highest temperature attained in all the conventional fixed- point cells constructed by preparing a crucible made of graphite and causing pure metals to be cast in this crucible is the copper point, 1085° C. This is because the melting of a metal having a melting point higher than this highest temperature in a crucible made of graphite induces dissolution of the graphite from the crucible and consequently lowers the purity of the metal and brings a decrease in the freezing point.
Since the temperature fixed points that are available for temperature scale realization are restricted to below the copper point of 1085° C. owing to the absence of a fixed-point cell capable of affording a fixed point exceeding the copper point, the interpolated temperature scale using four fixed points from the zinc point to the copper point and ranging from 400° C. to 1100° C. have been proposed (F. Sakuma and S. Hattori, “Establishing a Practical Temperature Standard by Using a Narrow-Band Radiation Thermometer with a Silicon Detector,” in Temperature: Its Measurement and Control in Science and Industry, 5 (Schooley et al. ed.), AIP, New York (1982) 421-427). It is, however, pointed out in the cited reference that when interpolated scale realized with fixed points not exceeding 1100° C. are extrapolated up to a high temperature exceeding 1100° C., the relevant accuracy is markedly degraded.
The dissemination of temperature scale exceeding the copper point, therefore, has been implemented by using a radiation thermometer of high accuracy as the instrument being calibrated, with the scale being set by the extrapolation method based on fixed-point calibration at the copper point and the evaluation of the optical property of the given thermometer. The scale uncertainty consequently attained is barely about 1° C. for 2000° C. because this realization involves tremendous labor in performing accurate evaluation of characteristic properties and on the other hand because the realization of temperature scale exceeding the copper point by extrapolation increases the uncertainty as the temperature departs from the copper point.
The strip lamp necessitates a correction dependent on the measurement wavelength because the emissivity of the tungsten ribbon used as the filament is not 1. The strip lamp, therefore, has allowed no easy utilization with high accuracy and has as well necessitated filling of an inert gas in the bulb of the lamp because a temperature exceeding 2000° C. induces vaporization of the tungsten ribbon. The gas strip lamp, however, is incapable of acquiring stable properties owing to the fact that the inert gas filled in the bulb causes convection.
The aforementioned method that uses a crucible made of alumina and utilizes the freezing point of palladium or platinum as the fixed point allows effective use for a short-term neasurement. Since alumina is vulnerable to thermal shock, the crucible made of alumina entails such a problem as sustaining fracture after repeated use. Thus, the aforementioned method that uses a crucible made of alumina is destitute of serviceability.
Even the calibration of a thermocouple by the palladium wire method entails the problem that the accuracy of interpolation is degraded because the reproducibility of temperature measurement is barely about 1° C. and also because even the copper point that is the nearest fixed point is separated by about 500° C. Thus, the aforementioned method of temperature calibration that implements the calibration of a thermocouple by the palladium wire method is incapable of acquiring fully sufficient accuracy.
The aforementioned method that realizes the fixed point by melting alumina in a crucible made of tungsten entails such problems as incurring difficulty in enclosing molten aluminum in a crucible of a horizontal type because of inferior workability of tungsten and in forming a blackbody cavity having emissivity close to 1 in the crucible because of low material emissivity of tungsten. The aforementioned method that uses a crucible made of tungsten, therefore, has low feasibility and is incapable of obtaining fully satisfactory accuracy.
The aforementioned method that uses a metal-metal eutectic alloy for the temperature fixed point is aimed at increasing the number of fixed points in the temperature range below the copper point. When this method is used above the copper point, however, the fixed point cannot be realized because the depression of the freezing point due to the dissolution of graphite from the crucible made of graphite is inevitable.
The aforementioned method that uses a metal-carbon eutectic alloy as the fixed-point material uses an alloy of invariably expensive noble metals as the fixed-point material in the temperature range from 1400° C. to 2300° C. It is, however, preferable to use as inexpensive a material as possible as the fixed-point material.
According to the aforementioned method that uses a metal carbide-carbon eutectic alloy as the fixed-point material, it is made possible to realize a titanium carbide-carbon eutectic point at temperatures in the neighborhood of 2750° C., for example. The titanium powder that is indispensable in the manufacture of a cell, however, is highly flammable and dangerous because it manifests high reactivity with oxygen and nitrogen in the air. Further, owing to the high reactivity mentioned above, the titanium powder of high purity itself is difficult to produce and is not readily available.
Further, the aforementioned method that uses a metal-carbon eutectic alloy and a metal carbide-carbon eutectic alloy for the fixed-point material entails the problem that the fixed-point material forms a solidified state possessing a structure such that the two layers are entangled in a lamellate shape or a flaky shape and the size of the solidified structure brings an effect on the energy equilibrium during the course of melting or freezing. Even in accordance with the aforementioned method that uses a metal-carbon eutectic alloy, therefore, the state of a sufficiently flat plateau cannot be obtained during the melting of the fixed-point material. (N. Sasajima, Y. Yamada, P. Bloembergen and Y. Ono, Proc. TEMPMEKO 2004, p. 196(2005)).
This invention has been aimed at overcoming the problems incurred by the conventional techniques as described above and realizing a fixed point in the temperature range exceeding the copper point with the object of accomplishing great improvement of the accuracy of the calibration of radiation thermometers, thermocouples, and all other thermometers of the kind used in a high temperature range.